Preparation of catalyst precursors by metal impregnation onto catalyst supports using various impregnation techniques is well known to those skilled in the art. The impregnated supports so obtained are then usually subjected to drying and calcination to provide catalyst precursors, and the precursors are then subjected to reduction to produce, finally, a catalyst.
EP-A-0 736 326 describes cobalt impregnated alumina based Fischer-Tropsch synthesis catalysts synthesized by means of aqueous slurry phase impregnation of a cobalt salt, for example cobalt nitrate hexahydrate, onto an alumina support, coupled with drying of the impregnated support, followed by direct fluidized bed calcination of the resultant impregnated support, to obtain a catalyst precursor, and then reducing the precursor to obtain the Fischer-Tropsch synthesis catalysts. These catalysts contain cobalt dispersed on the support. Higher cobalt loadings, which result in higher catalyst activities, can be achieved by repeating the cobalt salt impregnation step. However, this has a negative impact on the total process costs of catalyst fabrication and the time required to prepare the catalyst. Moreover, the maximum amount of metal that can be deposited per impregnation step is limited by the pore volume of the support.
Alternatively, suitable Fischer-Tropsch catalysts with high cobalt loadings can be prepared by mulling or kneading alumina (EP-A-0 455 307), silica (EP-A-0 510 771) or zirconia (EP-A-0 510 772) with a soluble or insoluble cobalt source. In that way, a paste can be obtained which is extruded, dried and calcined in order to obtain a catalyst or catalyst precursor. Especially in the case of an insoluble cobalt source, such as Co(OH)2, a high loading of cobalt may be obtained in this way. In this approach, the final shape of the support is determined during the catalyst preparation process. As a result, the mechanical strength and physical shape of the support cannot be pre-defined. Also, in order to obtain mechanically strong catalysts according to these known methods, the extrudates have to be calcined at relatively high temperatures. The drawback of high calcination temperatures is that the catalyst performance is adversely affected. An additional drawback of mulling or kneading is that organic delaminating agents are often needed. Such compounds give rise to an exothermic combustion with an exhaust of polluting volatile organic compounds.
Yet a further alternative method of obtaining high cobalt loadings, is the precipitation of an insoluble cobalt compound using an excess alkaline precipitating agent, subsequently deposited on a support by adding a soluble aluminium compound such as sodium aluminate (WO-A-2006/021754). The precipitation of a cobalt compound at a pH of >8 on a solid support such as Kieselguhr (WO-A-01/28962) by adding a base, has also been reported. In such cases, Co(NO3)2 is often used as a starting compound that is suggested to precipitated on to the support as a cobalt hydroxide species (Appl. Catal. A: Gen. 311 (2006), 146). The disadvantage of precipitation processes that require chemical treatment, such as addition of a base, is the production of waste such as salts. This necessitates excessive filtration or washing steps in the preparation process. In addition, such processes do not necessarily ensure sufficient mechanical catalyst strength to avoid down stream problems regarding attrition issues.
Thus, there is a need for hydrogenation catalysts, including Fischer-Tropsch catalysts, with high loadings of active catalyst component such as cobalt, obtained by a simple preparation process that allows mechanically strong pre-shaped supports to be used and that avoids or at least reduces the use of chemical treatments, such as addition of a base, or other disadvantages as described above.